Brick Texture On Extruded Cylinder Shows All Mortar Color
It's an UV map problem
You may have correctly unwrapped the cylinder (or used the default cylinder which comes already unwrapped) but unless you are using the brand new (2.90) 3D View Transform option, called Correct Face Attributes, which allows the transform tools to edit the UV’s and vertex colors of your mesh while you manipulate the model, you'll have to unwrap the new faces too.
The UV island for the interested faces is probably collapsed onto a single line in the UV editor, which falls in corrispondence if a mortar.